The present invention relates to polarization transforming optics, polarizing beam splitters (PBS's) and liquid crystal displays (LCD's) and more particularly to a polarization transforming optics, a PBS and a LCD which are suitable for applications in which polarized light is used as illumination light for a liquid crystal light valve.
Available as an LCD utilizing polarized light is, for example, a liquid crystal projector using a TN (twisted nematic) type liquid crystal light valve.
Conventionally, this type of apparatus uses a polarization transforming optics of the type in which only flux of unidirectional, linearly polarized light is derived from flux of unpolarized light from a light source by means of a polarizer such as a polarizer plate made of polyvinyl alcohol or PBS to illuminate the liquid crystal light valve, and the attainable utilization efficiency of light is 50% even at its maximum.
Contrary to this, an LCD has been proposed including a polarization transforming optics in which light flux from a light source is separated by using a PBS into two linearly polarized light flux beams heading in directions which are orthogonal to each other and the polarizing plane of at least one of the two linearly polarized light flux beams is rotated so that the direction of linear polarization of the one linearly polarized light flux beam may coincide with that of the other linearly polarized light flux beam. Then, the two light flux beams delivered from the polarization transforming optics are used as illumination light for a liquid crystal light valve to obtain a light utilization efficiency which is 50% or more.
An example of the conventional PBS will be detailed below. FIG. 10 is a perspective view of a conventional PBS 60. In the PBS 60, triangle pole prisms 61 and 62 each having right-angled isosceles triangular base are jointed together to form a cubic shape as a whole. Each of the triangle pole prisms 61 and 62 is the same as a so-called right-angled prism or triangular prism. Formed at a joint interface between the triangle pole prisms 61 and 62 is a PBS film 63 which is an optical multi-layer thin film.
A polarizing separation operation will be described by referring to an instance where flux of light 9 heading in the x direction of Cartesian coordinates shown in the figure impinges upon the PBS 60, on the assumption that the incident light flux 9 is unpolarized light containing equally a polarized component 9y in the y direction and a polarized component 9z in the z direction, and that the incident light flux 9 has a cross-sectional form which is square like each side of the cube. The incident light flux 9 reaches the PBS film 63 by which it is split into two beams of linearly polarized outgoing light flux 64 and 65 having polarizing directions which are mutually orthogonal. More specifically, the P polarized outgoing light flux 64 transmitting through the PBS film 63 and heading in the x direction contains only a y-direction polarized component 64y corresponding to the polarized component 9y of the incident light flux 9 and the S polarized outgoing light flux 65 reflected by the PBS film 63 and heading in y direction contains only a z-direction polarized component 65z corresponding to the polarized component 9z of the incident light flux 9. Of the two beams of outgoing light flux 64 and 65 heading in a plane parallel to the xy plane, the outgoing light flux 64 has the polarized component 64y which is parallel to the xy plane but the outgoing light flux 65 has the polarized component 65z which is vertical to the xy plane.
Therefore, in order to realize a polarization transforming optics by using the conventional PBS 60, there is needed, in addition to a mirror for deflecting and transforming the heading direction of at least one of the two beams of outgoing light flux 64 and 65 into the heading direction of the other, an optical element for rotating the polarizing plane of at least one of the two beams of outgoing light flux 64 and 65 to transform the linear polarization direction of the one outgoing light flux into that of the other outgoing light flux.
In connection with the conventional polarization transforming optics using the PBS, Japanese Patent Application Laid-open Nos. Sho 61-90584, Sho 63-168622, Sho 63-271313, Hei 2-39084, Hei 2-69715, Hei 2-189504, Hei 2-239219 or Eurodisplay '90 (Holland) Lecture No. 2.6, for example, disclose a proposed polarization transforming optics which uses a PBS, a 1/2 wave plate or two 1/4 wave plates and a mirror. The PBS takes part in polarizing separation, the 1/2 wave plate or two 1/4 wave plates take part in rotation of a polarizing plane and the mirror takes part in deflection of a heading direction.
Also, as disclosed in, for example, Japanese Patent Application Laid-open Nos. Sho 61-122626, Sho 61-126516, Sho 63-168626 or Hei 2-93580, a polarization transforming optics has been proposed which uses a PBS, a 1/4 wave plate and a mirror. The PBS takes part in polarizing separation, the 1/4 wave plate in rotation of a polarizing plane and the mirror in deflection of a heading direction.
Further, as disclosed in, for example, Japanese Patent Application Laid-open No. Sho 63-197913, Japanese Utility Model Application Laid-open No. Hei 1-88902, Japanese Patent Application Laid-open No. Hei 2-64613, The Institute of Electronic Information and Communication Engineers of Japan, 1st Year of Heisei, Autumn Nationwide Meeting Lecture, Summary C-34 or "Flat Panel Display", '91, pp 225-231, a polarization transforming optics has been proposed which uses a PBS and two or more mirrors. The PBS has the polarizing separation function and the two or more mirrors have the heading direction deflecting function and the polarizing plane rotating function.
It is to be noted that the use of a mirror or a prism for deflection of an optical axis and rotation of an image is known in the art. For example, Optical Engineering Handbook, pp 518-519 shows that an amici prism having two mirror surfaces functions to rotate an image. Since the polarizing plane rotates concomitantly with the image rotation, it is known that two or more mirrors have the function of rotating the polarizing plane.
However, the prior art polarization transforming optics using the PBS faces problems as below.
The polarization transforming optics using the PBS, the 1/2 wave plate or two 1/4 wave plates and the mirror, disclosed in, for example, the aforementined Japanese Patent Application Laid-open No. Sho 61-90584 includes, in addition to the PBS of a cubic form, one right-angled prism having half the volume of the PBS and a wave plate in the form of a flat plate. When the volume of the wave plate having a small thickness is neglected, the polarization transforming optics has a volume which is 1.5 times the cubic form of the PBS and so can be relatively compact. In the wave plate, however, the phase difference of birefringence has great dependency upon wavelength and hence the utilization efficiency of light is degraded when white light is incident on the wave plate. For example, the aforementioned Eurodisplay '90 (Holland) Lecture No. 2.6 shows that while the ideal light utilization efficiency is twice the conventional light utilization efficiency which is 50%, the actually obtained light utilization efficiency is only 1.34 to 1.5 times the conventional 50% light utilization efficiency. In addition, when the optics is applied to a projection type LCD, flux of light from a light source is very high in intensity of illumination and therefore light-proof capability of the wave plate is a matter of importance.
The polarization transforming optics using the PBS, 1/4 wave plate and mirror, disclosed in, for example, the aforementioned Japanese Patent Application Laid-open No. Sho 61-122626 includes, in addition to the cubic PBS, the 1/4 wave plate and a planar reflector. When the volume of the wave plate and planar reflector each having a small thickness is neglected, the polarization transforming optics has a volume which corresponds to one cube of the PBS and so can have a minimum size. However, because of the use of the wave plate as in the case of the just above-described prior art, there occur problems of degradation in light utilization efficiency and light-proof capability of the wave plate. Especially, in the aforementioned Japanese Patent Application Laid-open No. Sho 61-90584, only one of the two beams of synthesized light flux passes once through the wave plate but the other does not. Contrary to this, in the prior art of '26, one of the two beams of synthesized light flux passes once through the 1/4 wave plate once and the other passes through three times. Accordingly, in comparison with the prior art of '84, the degradation in the light utilization efficiency is aggravated. In addition, intensity of illumination light incident on the wave plate further increases, making the problem of light-proof capability of the wave plate more serious.
The polarization transforming optics using the PBS and the two or more mirrors, disclosed in, for example, the aforementioned Japanese Utility Model Application Laid-open No. Hei 1-88902 includes, in addition to the cubic PBS, four right-angled prisms each having a volume which is half the volume of the PBS. The rotation of the polarizing plane is effected using a total reflection surface of the right-angled prism, with the result that the light utilization efficiency is higher than that of the prior art using the wave plate and the problem of light-proof capability does not occur. But, in addition to the cubic PBS the four right-angled prisms each having a volume which is half the volume of the PBS are also used and consequently a total volume is three times the cubic volume, resulting in an increased size which is two to three times the size of the prior art using the wave plate. Accordingly, an LCD using the polarization transforming optics is also increased in size. Further, as the size of the prism increases, a large glass material is required for use as the prism, raising the cost. Especially, in order to obtain a PBS having an excellent polarization capability (contrast ratio) for incident white light, a glass material having a larger refractive index than that of BK7 must be used. But such a glass material is in general more expensive than BK7 and because of a large amount of the material being used, the cost is raised drastically.
The aforementioned Japanese Patent Application Laid-open No. Sho 63-197913 shows the polarization transforming optics using the PBS and the two mirrors. Assumptively, when right-angled prisms are used in place of the mirrors, two right-angled prisms each having a volume which is half the volume of the cubic PBS are used in addition to the PBS and a relatively small total volume results which is twice the cubic volume. But, two beams of light flux delivered from the polarization transforming optics are displaced from each other by a width of each light flux beam in both the longitudinal direction (z direction) and lateral direction (x direction) and there results a synthesized light flux having a cross-sectional form which is not rectangular, demonstrating unsuitability of the polarization transforming optics for an LCD using a rectangular liquid crystal light valve. Conceivably, the mirrors may be inclined to cause heading directions of the two light flux beams to be deflected inwards by the same angle, so that the two light flux beams intersect with each other to provide light suitable for illumination of the liquid crystal light valve. In this case, however, the displacement between the two light flux beams is large amounting up to about 1.41 times the width of each light flux and consequently the deflection angle must be increased, with the result that the incident angle to the liquid crystal light valve is increased to degrade the contrast performance. If a color separation/synthesis optical system of, for example, dichroic mirrors is interposed between the liquid crystal light valve and polarization transforming optics, the incident angle to these mirrors is also increased to degrade color purity and aggravate color shading.
In the polarization transforming optics of the aforementioned Japanese Patent Application Laid-open Nos. Sho 61-90584, Sho 61-122626 and so on, because of the use of the wave plate, the light utilization efficiency is degraded and the problem of light-proof capability is raised and in the polarization transforming optics of the aforementioned Japanese Utility Model Application Laid-open No. Hei 1-88902, Japanese Patent Application Laid-open No. Sho 63-197913 and so on, the two or more mirrors are needed in addition to the PBS. Especially, in an application to an LCD, the distance between optical axes of the two beams of light flux delivered from the PBS has to be reduced, thus requiring at least four mirrors. Accordingly, the size of the optical system is increased and the size of a glass material used therefor is also increased, raising the cost.